Mesogenic benzothiazole derivatives with a polar nitro substituent

Article abstract of DOI:10.1080/15421400802697616

Two new mesogenic homologous series containing 6-nitrobenzothiazole moiety at the terminus of the molecule, viz 2-(4'-n-alkoxyphenylazo)-6- nitrobenzothiazoles and 2-[4'-(4-n alkoxybenzoyloxy) phenylazo]-6- nitrobenzothiazoles were synthesized. The molecular structures of these new compounds of both the series have been characterized by elemental analysis, FTIR, and NMR spectral studies. The mesomorphic behavior has been studied using mainly polarized microscopy and in some cases differential scanning calorimetry as well. All the nine compounds of the series I are mesogenic and exhibit only the smectic A mesophase. In series II, all the twelve members synthesized exhibit enantiotropic mesomorphism. This series exhibits enantiotropic nematic mesophase from methoxy to n-heptyloxy derivatives. The smectic C mesophase commences from methoxy derivative, and persist up to n-butyloxy derivative, whereas the smectic A mesophase enters from the n-butyloxy derivative, and remains up to the last member synthesized. The mesomorphic properties of the present series are compared with other structurally related series to evaluate the effect of the benzothiazole ring and the polar nitro substituent on mesomorphism.

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Mesogenic Benzothiazole
Derivatives with a Polar Nitro
Substituent
A. K. Prajapati a & N. L. Bonde a
a Applied Chemistry Department, Faculty of
Technology and Engineering , The M. S. University of
Baroda , Kalabhavan, Vadodara, India
Published online: 18 Mar 2009.
To cite this article: A. K. Prajapati & N. L. Bonde (2009) Mesogenic Benzothiazole
Derivatives with a Polar Nitro Substituent , Molecular Crystals and Liquid Crystals,
501:1, 72-85, DOI: 10.1080/15421400802697616
To link to this article: http://dx.doi.org/10.1080/15421400802697616
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Mol. Cryst. Liq. Cryst., Vol. 501, pp. 72–85, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400802697616
Mesogenic Benzothiazole Derivatives with a Polar
Nitro Substituenty
A. K. Prajapati and N. L. Bonde
Applied Chemistry Department, Faculty of Technology and Engineering,
The M. S. University of Baroda, Kalabhavan, Vadodara, India
Two new mesogenic homologous series containing 6-nitrobenzothiazole moiety at
the terminus of the molecule, viz 2-(40-n-alkoxyphenylazo)-6-nitrobenzothiazoles
and 2-[40-(4’’-n alkoxybenzoyloxy) phenylazo]-6-nitrobenzothiazoles were synthe-
sized. The molecular structures of these new compounds of both the series have
been characterized by elemental analysis, FTIR, and NMR spectral studies. The
mesomorphic behavior has been studied using mainly polarized microscopy and
in some cases differential scanning calorimetry as well. All the nine compounds
of the series I are mesogenic and exhibit only the smectic A mesophase. In series
II, all the twelve members synthesized exhibit enantiotropic mesomorphism. This
series exhibits enantiotropic nematic mesophase from methoxy to n-heptyloxy
derivatives. The smectic C mesophase commences from methoxy derivative, and
persist up to n-butyloxy derivative, whereas the smectic A mesophase enters from
the n-butyloxy derivative, and remains up to the last member synthesized. The
mesomorphic properties of the present series are compared with other structurally
related series to evaluate the effect of the benzothiazole ring and the polar nitro
substituent on mesomorphism.
Keywords: 6-Nitrobenzothiazole; azomesogens; nematic; smectic A; smectic C
INTRODUCTION
The liquid crystalline properties of organic compounds are mainly
depends on the mesogenic core structure, it’s geometry, polarizability
ratio, lateral to terminal attractive force ratio, the length to breadth
ratio, etc. [1].
yThis article was presented at the International Conference on Liquid Crystal (ICLC-
2006) held at Department of Physics, University of Mumbai, Santacruz (E), Vidyanagari,
Mumbai during December 5–8, 2006.
Address correspondence to A. K. Prajapati, Applied Chemistry Department, Faculty
of Technology and Engineering, The Maharaja Sayajirao University of Baroda, P. O. Box
No. 51, Kalabhavan, Vadodara 390-001, India. E-mail: akprajapati@yahoo.co.uk.
72

Mesogenic Benzothiazole Derivatives
73
Until 1983, it was generally accepted that lateral substituent
reduce or diminish the mesogenic property of a compound, and the
extent of the effect depends on the size of lateral substituent by
disrupting the molecular packing. Many researchers studied the meso-
genic properties of lateral substituted homocyclic compounds, and
they have established the relation for the effect of different lateral
substituents on the mesogenic properties. Wide ranges of different
lateral substituents (e.g., -F, -Cl, -NO2, -CH3, -CF3) have been incorpo-
rated into many liquid crystal systems in different environments.
With the aim of regulating the parameters of the liquid crystal, the
meso state has been widely studied, and definite data have already
been accumulated on the change of properties of a liquid crystal
depending on the type and position of a substituent [2–4].
Liquid crystals containing a heterocyclic ring system have also been
attracting attention in their own right. These kinds of mesogens exhi-
bit interesting properties [5], such as lower symmetry and different
polarizability as compared to the homocyclic unit. In addition to this,
it possesses an additional permanent dipole moment, that affects the
magnitude and direction of the total molecular dipole moment without
increasing the molecular breadth [6].
Many mesogenic compounds containing a heterocyclic ring have
been reported in the literature. Single ring heterocyclic moieties such
as pyridine [6–8], furyl [6], substituted pyrimidine carboxylic acid [9],
thiazole [10], thiadiazole [11–14], and oxadiazole [13] are reported in
the literature. Barbera et al. [15] have synthesized 2-pyrazoline deri-
vatives and also studied the optical, NLO and mesogenic properties of
such materials. Highly polar thiophene based liquid crystals have also
been reported [16] in the literature. However, fused ring heterocyclic
derivatives, which are two condensed rings, are practically planar
and rigid molecule. Liquid crystals containing fused heterocyclic ring
system such as coumarin [17] mesogens containing azo central link-
age, thiazole[5,4-d]thiazoles [18], imidazo[2,1-b]-1,3,4-thiadiazoles
[19], and Isocoumarin [20] derivatives having central ester and amide
linkage exhibiting smectic C and nematic mesophase, and very high
mesophase transition temperatures are reported in the literature.
Esters of 7-decanoyloxychromone-3-carboxylic acid [21,22], flavone
[23] has also been reported. Recently Xuelong et al. [24] synthesized
and studied the mesogenic properties of benzo-2, 1-3-thiadiazole
derivatives.
Pavluchenko et al. [25] reported mesogens containing benzothizole
and benzoxazole with different central linkages and different lateral
substitution at different positions so as to evaluate the effect of struc-
tural changes on mesomorphic properties. They observed that the

74
A. K. Prajapati and N. L. Bonde
substitution at sixth position was thermally more stable as compared
to that of the fifth position on the benzothiazole ring. They have also
concluded that the 6-substitued benzothiazoles in comparison with
their benzoxazole analogs has greater mesophase transition tempera-
tures because of the difference in degree of aromatization of azole
cycles that is degree of conjugation of heteroatom p-electrons with
p-electrons of cycle.
Recently, we reported two mesogenic homologous series of each
containing 6-chloro benzothiazole [26] and 6-methoxybenzothiazole
[27] ring systems with azo and=or ester central linkage. In order to
study more about the effect of benzothiazole ring and nitro substituent
on mesomorphic properties in such systems, we have synthesized two
new extensive mesogenic homologous series of benzothiazole deriva-
tives containing 6-nitro substituents.
EXPERIMENTAL
Characterization
The structures of the compounds were confirmed by IR spectroscopy
via KBr pellet, using a Shimadzu IR-408 spectrophotometer. 1H
NMR spectra were obtained with a Bruker Dpx 200 spectrometer or
Bruker 300, using tetramethylsilane (TMS) as an internal reference
standard. The chemical shifts are quoted as d (parts per million) down-
field from reference. CDCl3 was used as a solvent. The liquid crystal-
line properties of the mesogens were investigated on Leitz Laborlux
12 POL microscope provided with heating stage. The enthalpies of
transitions reported as J=g were determined from thermograms
obtained on a Universal V3 ꢀ 0G TA instrument adopting rate of
5ꢁC=min. The calorimeter was calibrated using pure indium as
standard. The purity of the final products was confirmed by thin layer
chromatography and microanalysis performed on a Perkin–Elmer
series 2400 elemental analyzer.
Materials
4-Hydroxybenzoic acid, 4-methoxybenzoic acid, 1-bromoalkanes,
thionyl chloride, anhydrous potassium carbonate, sodium nitrite,
4-nirtoaniline, potassium thiocyanate, bromine liquid and phenol,
etc. were used as received. Column chromatography was done on
Acme’s Silica Gel (100–200 mesh). Solvents were dried and distilled
prior to use.

Mesogenic Benzothiazole Derivatives
75
Synthesis
The synthetic route to series I and II is illustrated in Scheme 1.
2-Amino-6-nitrobenzothiazole [2]
2-Amino-6-nitrobenzothiazole was prepared from 4-methoxyaniline
[1] by the reported method [28]. Crystallization from 1:1 aqueous etha-
nol afforded pure yellow needles of [2]. Yield: 73%. M.P.: 246ꢁC
(Reported [28] m.p.: 247–249ꢁC).
2-(40-Hydroxyphenylazo)-6-nitrobenzothiazole [3]
This was synthesized by diazotization of compound [2] and coupling
with phenol by following the method reported by Johari and Varshney
[29]. The dye thus obtained was crystallized several times using alco-
hol until constant melting point was obtained. Yield: 53%. M.P.:
203ꢁC. Elemental analysis found
C 51.74, H 3.08, N 18.34,
C13H8N4O3S requires C 52.00, H 2.69, N 18.66%. The IR spectrum
showed a broad peak of intermolecularly hydrogen bonded, phenolic
-OH between 3200–3600 cmsꢂ1. Other signals observed were at 3046
(nC-H, aromatic), 2922 (nC-H, aliphatic), 1603 (-N ¼ N-), 1496, 1056
(Benzothiazole), 1027, 827, 635 (C-S-C) cmꢂ1
.
SCHEME 1 Synthetic route to series I and II. Reagents and conditions: (i)
KSCN, Br2, AcOH; (ii) NaNO2, H2SO4, 0–5ꢁC; (iii) phenol, 10% NaOH (aq.),
0–10ꢁC; Hcl; (iv) RBr, K2CO3, acetone; Series I: R ¼ -CnH2nþ1, n ¼ 4–8, 10,
12 and 16; (v) RBr, KOH, Ethanol; (vi) SOCl2 (Excess); (vii) pyridine; 0–
10ꢁC; Hcl; Series II: R ¼ -CnH2nþ1, n ¼ 1–8, 10, 12, 14, and 16.

76
A. K. Prajapati and N. L. Bonde
2-(40-n-Alkoxyphenylazo)-6-nitrobenzothiazoles [I]
These were synthesized by alkylation of dye [3] using the reported
method of Vyas and Shah [30]. 2-(40-Hydroxyphenylazo)-6 nitro
benzothiazole (0.1 mol), appropriate 1-bromoalkane (0.12 mol) and
anhydrous potassium carbonate (0.15 mol) were added to dry acetone
(60 ml). The reaction mixture was refluxed on a water bath for eight
to ten hours. Completion of the reaction was checked by thin layer
chromatography (TLC) (10% ethyl acetate: hexane). The reaction mix-
ture was added to ice-cold water. The crude solid product thus
obtained was triturated with cold 5% aqueous sodium hydroxide solu-
tion for 30 min so as to remove unreacted azo dye and was washed with
water. The product obtained was purified by column chromatography
using silica (100–200 mesh size) and 3% ethyl acetate-petroleum ether
(60–80) as eluent. The compound obtained was finally crystallized
using ethanol. Yield: 68–73%. The elemental analysis of all the com-
pounds of series I was satisfactory, and results are listed in Table 1.
TABLE 1 Elemental Analysis for Series I and II Compounds
% Required (% Found)
R¼-CnH2nþ1
Compound
n
Formula
C
H
N
Series I
1
2
3
4
5
6
C17H16N4O3S 57.29 (57.43) 4.52 (4.36) 15.72 (15.59)
C18H18N4O3S 58.36 (58.08) 4.90 (4.74) 15.12 (15.23)
C19H20N4O3S 59.36 (59.61) 5.24 (5.08) 14.57 (14.18)
C20H22N4O3S 60.28 (60.36) 5.56 (5.93) 14.06 (14.27)
4
7
5
6
7
8
8
C
C
21H24N4O3S 61.14 (60.58) 5.86 (5.67) 13.58 (13.82)
23H28N4O3S 62.70 (62.25) 6.41 (6.80) 12.72 (13.10)
10
12
14
16
C25H32N4O3S 64.08 (63.82) 6.88 (6.06) 11.96 (11.54)
C27H36N4O3S 65.29 (65.60) 7.31 (7.14) 11.28 (11.58)
C29H40N4O3S 66.38 (66.73) 7.68 (7.40) 10.68 (10.43)
9
Series II
10
11
12
13
14
15
16
17
18
19
20
21
1
2
3
4
5
6
7
8
10
12
14
16
C21H14N4O5S 58.06 (58.43) 3.25 (2.80) 12.90 (13.41)
C22H16N4O5S 58.92 (59.06) 3.60 (3.88) 12.49 (12.07)
C23H18N4O5S 59.73 (59.41) 3.92 (3.67) 12.11 (12.45)
C24H20N4O5S 60.49 (60.12) 4.23 (4.59) 11.76 (11.98)
C25H22N4O5S 61.21 (61.59) 4.52 (4.83) 11.42 (10.95)
C26H24N4O5S 61.89 (62.28) 4.79 (4.38) 11.10 (11.32)
C27H26N4O5S 62.53 (62.21) 5.05 (5.39) 10.80 (11.25)
C28H28N4O5S 63.14 (63.57) 5.30 (4.94) 10.52 (10.87)
C
30H32N4O5S 64.27 (63.77) 5.75 (6.06) 09.99 (09.63)
C32H36N4O5S 65.28 (65.36) 6.16 (6.37) 09.52 (09.84)
34H40N4O5S 66.21 (65.94) 6.54 (6.13) 09.08 (09.40)
C36H44N4O5S 67.05 (67.41) 6.88 (7.30) 08.69 (08.82)
C

Mesogenic Benzothiazole Derivatives
77
1
IR and H NMR spectral data of n-tetradecyloxy derivative as repre-
sentative member are given below.
2-(40-n-Tetradecyloxyphenylazo)-6-nitrobenzothiazole
IR spectrum (KBr) nmax=cmꢂ1: 3051(nC-H, aromatic), 2915(nC-H, alipha-
tic), 2849, 2243, 1603 (-N ¼ N-), 1573, 1473, 1436, 1256 (Aryl ether),
1
1058 (Benzothiazole), 1029, 849, 636(C-S-C) cmꢂ1. H NMR spectrum
(CDCl3, 200 MHz): d 8.81 (d, J7,5 ¼ 2.1 Hz, 1H, ArH at C-7), 8.35–8.41
(dd, J5,7 ¼ 2.1 Hz, J5,4 ¼ 9.2 Hz, 1H, ArH at C-5), 8.23 (d, J ¼ 9.2 Hz, 1H,
ArH at C-4), 8.10 (d, J ¼ 8.6 Hz, 2H, ArH at C-20 and C-60), 7.06 (d,
J ¼ 9.0 Hz, 2H, ArH at C-30 and C-50), 4.11 (t, 2H, Ar-O-CH2), 1.85
(m, 2H, Ar-O-C-CH2), 1.27–1.55(m, 22H, 11 ꢃ CH2), 0.88 (t, 3H, CH3).
4-n-Alkoxybenzoic Acids [5] and 4-n-Alkoxybenzoyl
Chlorides [6]
These were synthesized by the modified method of Dave and
Vora [31].
2-[40-(400-n-Alkoxybenzoyloxy) Phenylazo]-6-
nitrobenzothiazoles [II]
The compound [3] (0.02 mol) was dissolved in dry pyridine (5 ml),
and a cold solution of 4-n-alkoxybenzoyl chloride [6] (0.02 mol) in dry
pyridine was added with stirring in an ice bath. The mixture was
allowed to stand overnight at room temperature. It was acidified with
cold 1:1 aqueous hydrochloric acid. The solid obtained was separated,
dried, and triturated by stirring for 30 min with 5% aqueous sodium
hydroxide solution and was washed with water. The insoluble mass
separated was crystallized several times from acetic acid until
constant transition temperatures were obtained. Yield 59–64%. The
elemental analysis of all compounds of series II was satisfactory,
1
and results are listed in Table 1. IR and H NMR spectral data of n-
tetradecyloxy derivatives as representative member are given below.
2-[40-(400-n-Tetradecyloxybenzoyloxy) Phenylazo]-6-
nitrobenzothiazole
IR spectrum (KBr) nmax=cmꢂ1: 3091 (nC-H, aromatic), 2921 (nC-H, ali-
phatic), 2870, 1728 (-COO-), 1605 (-N ¼ N-), 1578, 1514 (Ar-NO2),
1470, 1345, 1039 (Benzothiazole), 894, 848, 646 (C-S-C). 1H NMR spec-
trum (CDCl3, 200 MHz): d 8.87 (d, J ¼ 2.3 Hz, 1H, ArH at C-7) 8.41 (dd,
J5,7 ¼ 2.3 Hz, J5,4 ¼ 8.9 Hz, 1H, ArH at C-5), 8.31 (d, J ¼ 8.9 Hz, 1H ArH
at C 4), 8.16–8.25 (m, 4H, ArH at C-20, C-60, C-200, and C-600), 7.51 (d,

78
A. K. Prajapati and N. L. Bonde
J ¼ 8.8 Hz, 2H, ArH at C 30 and C-50), 7.01(d, J ¼ 9 Hz, 2H, ArH at C-300
and C-500), 4.08 (t, 2H, Ar-O-CH2), 1.85 (m, 2H, Ar-O-C-CH2),
1.22–1.58 (m, 22H, 11 ꢃ CH2), 0.90 (t, 3H, -CH3).
RESULTS AND DISCUSSION
Microscopic Observation and DSC for Series I and II
The mesophase exhibited by series I and II compounds are identified
by examining the thin film of a sample sandwiched between a glass
slide and cover slip. On cooling the isotropic liquid in an ordinary slide,
polygonal or focal-conic textures characteristic of SmA phase were
observed for compounds of series I. Compounds of series II, on cooling
from isotropic liquid, methoxy to n-heptyloxy derivatives exhibit the
schlieren texture of the nematic phase, which on further cooling for
n ꢄ 3 exhibit the schlieren texture of SmC mesophase, whereas for
n ꢅ 4 showed the focal conic texture of SmA mesophase. Only in case
of n-butyloxy derivative, on further cooling the focal conic texture of
SmA changes to the schlieren texture of SmC mesophase. Higher
members (n ꢅ 8) on cooling from isotropic liquid showed only the
focal-conic texture characteristic of SmA mesophase. Transition
temperatures obtained for both the series are recorded in Tables 2
and 3, respectively (enantiotropic phase temperatures are recorded
in the heating cycle).
Calorimetry is a valuable method for the detection of phase transi-
tion. Conclusions may be drawn concerning the nature of the phases,
which participate in the transition with the help of calorimetric
studies. In the present study, enthalpies of few compounds of series
I and II were measured by differential scanning calorimetry. Data
TABLE 2 Transition Temperatures (ꢁC) of the Series I Compounds
Compound
no.
R¼-CnH2nþ1
n
Cr
SmA
I
1
2
3
4
5
6
7
8
9
4
5
6
7
8
10
12
14
16
.
.
.
.
.
.
.
.
.
165
169
163
160
160
159
156
151
149
.
.
.
.
.
.
.
.
.
201
204
200
198
195
193
194
196
196
.
.
.
.
.
.
.
.
.

Mesogenic Benzothiazole Derivatives
79
I
TABLE 3 Transition Temperatures (ꢁC) of the Series II Compounds
Compound no. R ¼ -CnH2nþ1
n
Cr
SmC
SmA
N
10
11
12
13
14
15
16
17
18
19
20
21
1
2
3
4
5
6
7
8
10
12
14
16
.
.
.
.
.
.
.
.
.
.
.
.
136
134
126
121
148
146
147
161
154
151
155
142
.
.
.
.
—
—
—
—
—
—
—
—
266
261
258
151
—
—
—
—
.
.
.
.
.
.
.
—
—
.
.
.
.
.
.
.
—
—
—
—
—
326
325
317
314
309
308
303
—
.
.
.
.
.
.
.
.
.
.
.
.
—
255
263
276
282
294
285
277
266
263
—
—
—
—
—
—
—
—
.
.
—
—
—
are recorded in Table 4. Enthalpy values of the phase transitions agree
well with the existing literature value [32], which has helped in
further confirmation of the mesophase type.
The Phase Behaviour of Series I and II
Series I: 2-(40-n-Alkoxyphenylazo)-6-nitrobenzothiazoles
All the nine compounds of series I are mesogenic and exhibit only
the SmA mesophase. The transition temperatures are recorded in
Table 2. The plot of transition temperatures against the number of
carbon atom in the alkoxy chain (Fig. 1) show falling tendency for
SmA-I transition and also exhibit usual odd–even effect for lower
members. Gray [1] has explained that the terminal intermolecular
attractions are playing a part in determining the Sm-I transition
temperatures, i.e., the destruction of the smectic molecular order is
determined by the fact that the terminal attractions are becoming
TABLE 4 DSC Data for Series I and II Compounds
Series
I
n
Transition
Peak temp. (ꢁC)
DH=Jgꢂ1
DS=Jgꢂ1 Kꢂ1
8
Cr-Sm A
Sm A-I
Cr-Sm A
Sm A-I
Cr-Sm A
Sm A-I
159.4
193.5
152.1
195.7
155.9
—
15.64
0.78
22.64
01.22
0.03616
0.0017
0.0532
0.0026
0.0438
—
14
14
II
15.00
y
yEnthalpy could not be measured.

80
A. K. Prajapati and N. L. Bonde
FIGURE 1 The phase behavior of series I.
weaker, so allowing partial interpenetration of the layers to occur
more easily as the alkyl chains grow longer, which diminishes the
SmA-I transition. This explains the falling SmA-I transition tempera-
ture curve. Cr-SmA transition exhibit falling tendency as the series
ascended except the ethyloxy derivative.
Series II: 2-[40-(400-n-Alkoxybenzoyloxy) Phenylazo]-6-
nitrobenzothiazoles
All the 12 members synthesized exhibit enantiotropic mesomorph-
ism. The series exhibits enantiotropic nematic mesophase from meth-
oxy to n-heptyloxy derivatives. The SmC mesophase commences from
the methoxy derivative and persists up to n-butyloxy derivative
whereas the SmA mesophase begins from the n-butyloxy derivative,
which remains up to the last member synthesized. Transition tem-
peratures were recorded in Table 3. The plot of transition tempera-
tures against number of carbon atoms in the alkoxy chain (Fig. 2)
shows a smooth falling tendency for N to I, SmC to N, SmA to I transi-
tions. The N to I transition also exhibits the usual odd–even effect for
lower members. The SmA to N transition temperatures exhibit a ris-
ing tendency as series ascended.
Mesogenic Properties and Molecular Constitution
There is close relation between mesomorphism and molecular consti-
tution of organic compounds. Hence, transition temperature and
mesophase length; measures of mesomorphism can be correlated with
the molecular constitution of the compounds.

Mesogenic Benzothiazole Derivatives
81
FIGURE 2 The phase behavior of series II.
Table 5 shows the comparison of transition temperatures, range
(width) of mesophase phase, and thermal stabilities and molecular
structures of representative compounds 7 (n ¼ 12) and 16 (n ¼ 12) of
the present series I and II with other structurally related compounds
A [26], B [27], C [33], D [26], E [27], and F [33] reported in the litera-
ture. Both compounds 7 and A exhibit an enantiotropic smectic
mesophase. The smectic mesophase range of compound 7 is lower by
54ꢁC, whereas the smectic thermal stability is higher by 22ꢁC than
compound A. Reference to molecular structure indicates that both
compounds have different substituent at the sixth position of ben-
zothiazole moiety. Compound 7 has the -NO2 group, whereas com-
pound A has the -Cl group at the sixth position of benzothiazole
moiety. The higher smectic thermal stability of the mesogen 7 may
be because of the C-NO2 bond, which has greater polarizability com-
pared to the C-Cl bond [1]. Gray [1] has explained that the dipole
moment is an important factor as far as relative thermal stability of
smectic phase is concerned, and the size effect is not prominent in such
cases.
Table 5 indicates that compound B exhibits an enantiotropic
nematic mesophase along with a monotropic SmA mesophase, where-
as compound 7 exhibits only an enantiotropic SmA mesophase. The
smectic mesophase length and thermal stability of compound 7 are
higher by 31ꢁC and 86ꢁC, respectively than compound B. This may
be due to the presence of C-NO2 bond, which increases the molecular
polarizability and possibly also the molecular dipolarity as compared
to less dipolar -OCH3 group. This is also reflected in comparison of
compound A with compound B. The smectic mesophase length and
smectic thermal stability of compound A is higher by 85ꢁC and 64ꢁC,

82
A. K. Prajapati and N. L. Bonde
TABLE 5 The Comparison of Mesophase Length (ꢁC), Thermal Stabilities
(ꢁC), and Molecular Structure of Compounds 7, 16, 25, 37, 49, 61, A, and B
Mesophase length
Thermal stability
Compound
no.
Commencement of
smectic phase
N
Sm
N
Sm
7
—
—
38
92
7
—
—
194
172
108
—
C4
C5
C12
—
A
B
C
16
D
E
F
22
—
133
—
—
—
126
149
—
—
277
264
—
C1
C1
—
04
123
68
268
254
201
23
112
C10
respectively, than compound B because of the presence of the more
polar lateral -Cl group in compound A, whereas compound B has the
less polar -OCH3 group at the lateral position.

Mesogenic Benzothiazole Derivatives
83
Table 5 shows that compound C is nonmesogenic, whereas com-
pound 7 is mesogenic and exhibits an enantiotropic smectic meso-
phase. Reference to the molecular structure shows that compound C
differs only at one terminus from compound 7. Compound 7 has 6-
nitrobenzothiazole ring at the terminus instead of naphthalene ring
of compound C. Presence of lateral -NO2 group in compound 7,
increases the overall molecular breadth. Gray [1] has explained that
increase in the breadth of the molecule reduces both nematic and
smectic mesophase thermal stability, but due to the presence of term-
inal benzothiazole ring which increases overall polarizability of the
molecule with lowering the symmetry of the molecule as compared
to phenyl=naphthyl derivatives and provide higher thermal stability
[25]. Moreover, the polar nature of the nitro group also increases the
overall polarizability of the molecules. All these factors are responsible
for the mesogenic behavior of compound 7. The same observation
reflected in the comparison of compound A and B with compound C.
Reference to Table 5 indicates that, both compound 7 and 16 exhi-
bits the smectic A mesophase. The smectic mesophase length and
smectic thermal stability of compound 16 is higher by 88ꢁC and
83ꢁC, respectively, than compound 7. This is because of the greater
molecular length of compound 16 provided by the additional aromatic
ring and central ester linkage. As a result of its enhanced anisotropy of
polarizability and increase in the intermolecular cohesive forces that
are responsible for the wider mesophase length and higher thermal
stability of smectic mesophase. This is also observed during compari-
son of compound A with compound D, and compound B with
compound E. The mesophase length and thermal stability of com-
pound D are higher by 57ꢁC and 92ꢁC, respectively, than that of com-
pound A. Compound D also exhibits the nematic mesophase, due to
the enhanced polarizability and increase in intermolecular cohesive
forces, which are also responsible for this induction of the nematic
mesophase. The nematic mesophase range and nematic thermal
stability of compound E are higher by 101ꢁC and 121ꢁC, respectively,
than compound B.
Reference to Table 5 shows that the smectic mesophase range and
smectic phase thermal stability of compound 16 are higher by 103ꢁC
and 165ꢁC than compound F. The absence of the nematic mesophase
for compound 16 may be because of the polar -NO2 group, which
diminishes the nematic mesophase. Overall, compound 16 has greater
mesophase thermal stability. By reference to the molecular structure
of both compounds, it can be observed that both compounds have a
fused ring system at the terminus of the molecule, but the compound
16 has 6-nitrobenzothiazole ring at the terminus which increases the

84
A. K. Prajapati and N. L. Bonde
overall polarizability of the molecule [25]. Although the compound 16
has a lateral nitro substituent, the effect pronounced by lateral substi-
tuent is eliminated by the presence of heterocyclic benzothiazole ring.
In addition to this, the polar nature of lateral nitro group present in
compound 16 also increases the polarizability of the molecule this, also
assists to increase the thermal stability of the compound. This is also
reflected in the comparison of compound D and compound E with com-
pound F. Table 5 indicates that the smectic mesophase length and
thermal stability of compound D is higher by 126ꢁC and 152ꢁC, respec-
tively, than compound F, and the nematic mesophase length and ther-
mal stability of compound E are higher by 55ꢁC and 53ꢁC, respectively,
than compound F.
CONCLUSION
Two new mesogenic homologous series containing 6-nitrobenzothia-
zole moiety at the terminus of the molecule were synthesized. The
mesomorphic properties exhibited by the present series I and II show
that the members of series II exhibit higher mesophase thermal stabi-
lity as compared to those of series I due to the presence of an addi-
tional phenyl ring and a central ester linkage. All the derivatives of
both the present mesogenic homologous series exhibit higher meso-
phase thermal stabilities as compared to the naphthalene analogue
having no lateral substituent. It was also observed that the nitro sub-
stituent is more conducive to the generation of smectic mesophase as
compared to the chloro and the methoxy substituent.
ACKNOWLEDGMENTS
The authors are thankful to the Dean, Faculty of Technology and
Engineering, and Head, Applied Chemistry Department, for providing
the research facilities.
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